As can be seen from Figure REF , the first step in the simulation is providing the input files - namely array layout file, signal model, and/or foreground model. The layout file should have the coordinates of each antenna/station. For CASA, the local tangent coordinates are used, while OSKAR accepts both local tangent and WGS84 coordinates. Additionally, aperture arrays like SKA have a set of dipoles in each station/tile (whose combined response is the “station beam", equivalent to the primary beam for each telescope in case of a traditional dish array). Thus, OSKAR also requires separate layout files for each dipole in each station of the aperture array. The next input parameter is a model of the 21-cm signal. There are different approaches like radiative transfer [1]}, [2]}, semi-numerical methods [3]}, [4]}, ray tracing [5]}, radiation magnetohydrodynamics [6]} etc to generate the 21-cm ionization fields, brightness temperature distributions, perturbed fields etc. However, semi-numerical techniques remain the most popular method due to their computational efficiency and reasonable accuracy. Currently, we have tested the pipeline with two popular publicly available semi-numerical software for generating 21-cm signal - 21cmFASThttps://github.com/21cmfast/21cmFAST [3]}, [8]} and ReionYugahttps://github.com/rajeshmondal18/ReionYuga [9]}, [4]}, [11]}. The brightness temperature of the 21-cm signal is generated with a specified box length, grid size, and redshifted range. To “observe" this signal, it has to be associated with a world coordinate system. We calculate the angular size corresponding to the box size at the redshift of interest and the frequency range corresponding to the redshift range. Then using the RA, Dec of the phase center, a dummy image using the frequency and angular size information is generated, where the brightness temperature is projected (after conversion to Jy beam\(^{-1}\) ). This is used as the input signal. For the point source foregrounds, a source catalogue containing RA, Dec, and flux density of the sources is required. Optionally, reference frequency, source size, polarized intensity, and spectral index can also be included. In case of OSKAR, this is a simple ASCII catalogue. But for CASA, one needs to generate a componentlist, which is a source list in CASA specified format See https://casa.nrao.edu/docs/casaref/componentlist-Tool.html#x237-2380001.2.1. Foreground model can include any source catalogue generated from observations as well as semi-analytical simulations like T-RECS [12]}.
